Let's see...singwell's question...let me mind my p's and q's on this one.... (so you can use it to indicate a missing letter, possessive form of nouns, and plurals of lowercase letters)

Excuse me, but I'll have you know Apostrophe Woman and I just so happen to be excellent friends. We have talked about this over coffee many times and both of us agree that this is the best way to explain it. Apostrophe- Punctuation use First there is possesive apostrophe. When to use it? When someone (or something) has posession of some sort. -That is Mary's pencil. The apostophe is placed after the ending, and after, the letter "s" is placed. For words that already end in "s," just the apostrophe is used. -That was Mrs. Jones' idea. *His, hers, yours, theirs, whose, and its all do not need apostophes. Contractions are also another thing apostrophes are used for. You use them when you are combining the two words. -I'm, we're, can't, won't (etc...) And sometimes, apostrophes are used when you want to shorten a word. -Gov't -------------------------------------------------- Apostrophe- Literary device use An apostrophe in terms of literary devices is when the speaker interrupts the discussion to directly address a person or object given personal characteristics. They can be present or absent, it doesn't really matter, as long as they are being specifically addressed. We use this one when there is a strong sense of emotion building up, and the speaker eventually begins to let it all out and vent, making the apostrophe happen. ((A.P English has proved itself. For once.)

Apostrophe's are for possessives and word contractions. Use them when you want your scripted dialogue to should like real speech.